The function of a masking film is to form a protective cover or coating to prevent scratching, scuffing, and distortions of the smooth surface to which it is applied. This protective cover or coating is necessary for shipment and during use of the product having the smooth surface.
Previously, conventional corona treated film and/or adhesive coated masking paper have been used for masking applications upon rigid or hard, but still pliable, smooth surfaces of plastics especially polycarbonates. Polycarbonate sheet which typically is rigid and over 10 mils thick is usually masked with an adhesive backed paper. By contrast, polycarbonate film which typically is flexible and less than or equal to 10 mils thick is usually masked with a polyethylene film.
In masking polycarbonate film, two different types of polyethylene film are used to protect the sides of the polycarbonate film. One side may be covered with a one mil, corona treated, non-embossed, low slip, low density polyethylene film. The other side may be covered with a 3 mil, low density polyethylene film which has been coated with a heat activated adhesive. Frequently, polycarbonate film is sold by the manufacturer to intermediate operations which print upon the polycarbonate film prior to shipment to an end user. The polyethylene film is used as a protective layer both before and after the polycarbonate sheet is printed. The 1 mil layer is normally removed immediately before printing whereas the adhesive coated, 3 mil, low density layer remains on the polycarbonate throughout the printing process and is only removed by the end user. Normally in the manufacturing process, both polyethylene film layers are applied to the polycarbonate film simultaneously shortly after it is extruded. The polyethylene is applied with a nip roll when the polycarbonate sheet is between 100.degree. and 200.degree. F. The temperature varies with the thickness of the sheet.
By contrast, polycarbonate sheeting normally utilizes a paper based masking for protection on both sides of the sheet. The paper uses a heat activated acrylic adhesive to effect its adhesion to the polycarbonate. Frequently, humidity from the air permeates the paper and causes delamination between the paper and the adhesive. This allows the adhesive to remain on the polycarbonate sheet after the paper mask is removed. The adhesive then must be removed from the polycarbonate with a solvent.
Various problems exist with polyethylene films currently used to mask polycarbonate films or sheets. One problem is that often, a different polyethylene film is used for each surface or side of the polycarbonate film, causing increases in film inventory and complication of logistics and production methods. These increases and complications are a result of handling two types of film rather than one. Another problem is that currently used one mil, low slip, low density polyethylene film has been known to completely laminate to the polycarbonate film to which it is applied. This lamination renders the polycarbonate film essentially useless.
Another problem stems from the fact that the polyethylene film is received from the manufacturer and applied to the polycarbonate film from a roll. Polyethylene film blocks on the roll and develops hard wrinkles. These hard wrinkles are a problem since they do not come out of the polyethylene film prior to application on the polycarbonate film. These wrinkles will permanently distort the pliable surface of the polycarbonate film and render it useless for its desired end uses.
Still another problem arises due to application of the adhesive coated, 3 mil, low density polyethylene film itself which adheres to the polycarbonate film through the use of a heat activated adhesive. This adhesive can leave a residue on the polycarbonate film which is also undesirable.
Problems also exist with the adhesive paper mask used to protect the polycarbonate sheeting from scratches and dents during shipping and handling. The paper with adhesive is applied to the polycarbonate sheet at temperatures ranging from 100.degree.-320.degree. F. Frequently, humidity from the air permeates the paper and causes delamination. Similar problems exist for acrylic, glass, polished or painted metals and other items which have a rigid, smooth surface.
These currently used films of the related art will now be explained in more detail.
The one mil, low slip, low density polyethylene film is a corona treated type of masking film. Corona treatment is the utilization of an electrostatic discharge to increase the film's adhesion level. The electrostatic discharge causes surface oxidation of the film whereby the non-polar saturated hydrocarbon surface becomes more chemically reactive to polar surfaces which, in effect, increases the adhesion level of the non-polar surface. Currently used masking film is non-embossed and relies on a very narrow window of corona treatment to facilitate the films adhesion to a smooth surface. The corona treatment process cannot be controlled precisely enough to insure production is maintained within the desired window. When there is too little corona treatment, the masking film will not adhere to a smooth surface. When there is too much corona treatment, the masking film will either stick to itself and block within the roll, or the film will laminate to the smooth surface to which it is applied and become unremovable.
Corona treated masking films have a relatively high surface coefficient of friction (greater than 0.5). These films are extremely difficult to wind without inducing hard wrinkles which will distort a pliable, smooth surface upon application. Corona treated masking films normally have large and numerous gels and carbon specks which tend to dimple or distort the smooth surface of the substrate being protected. Gels are defined as unmelted polyethylene particles which range from a barely visible size up to larger than a pencil point.
Consequently, these masking films rely on a chemical differential slip where the chemical additive blooms or migrates to the outside surface of the masking film. While this prevents blocking or sticking of the masking film as it is unwound from the roll, the slip normally impairs the film's adhesion to a smooth surface rendering it less desirable as a masking film.
The aforedescribed adhesive coated, 3 mil, low density, polyethylene film is an example of an adhesive masking film. These films are non-embossed and affect their adhesion from a surface applied adhesive. The disadvantage of this type of film is that the adhesive residue is frequently left on the smooth surface following removal of the mask. Solvents must be used to remove this residue. At times the adhesive is too aggressive (adheres with great strength) which virtually makes the masking film unremovable. Similar to corona treated maskings, the adhesive masking films also are subject to hard wrinkles and gels.
While one-sided, matte embossed, polyethylene films are in existence, they are used as photopolymer cover sheets. Most photopolymers have the consistency of jelly. The photopolymers are coated upon a base of polyester film which acts as a protective carrier sheet. The one-sided, matte embossed, polyethylene film is then applied with the smooth, non-embossed side coming in contact with the exposed photopolymer surface and acting as a cover sheet. The net result is a three layered sandwich consisting of polyester film/photopolymer/matte polyethylene which is then wound up in rolls of varying widths of up to 60 inches and diameters of 10-12 inches.
Currently, photopolymers are made using two different types of polyethylene cover sheet films. If a photopolymer coating is to be thin, a one-sided matte polyethylene film is used with the smooth side applied to the photopolymers. If the photopolymer layer is to be thicker, a two-sided matte polyethylene film would be used. The difference between one-sided and two-sided matte embossed is that in one-sided matte embossed, one side is smooth and the opposite side is matte embossed while in two-sided matte embossed, both sides of the film are matte embossed. A two-sided embossment is made by a matte engraved roll that compression embosses the molten polyethylene between the embossing roll and a rubber covered well. A one-sided matte is produced by compression of molten polyethylene between a polished chrome roll and a rubber roll.
Photopolymers are used to make circuit boards. Prior to applying photopolymer to a dielectric board, the matte embossed polyethylene is removed from the three layer sandwich previously described. The exposed surface of the photopolymer is then laid upon the dielectric board. The other side of the photopolymer retains the polyester film carrier sheet. A photo tool is then placed on top of the polyester film and the entire multilayer substrate is then exposed to a light source.
The areas of the photopolymer which were exposed to light, harden and form a protective covering in the dielectric board. The dielectric board, with the exposed photopolymer, is then subjected to numerous chemical baths and rinses with the end result producing a printed circuit board.